Cytoskeleton

• Actin dynamics for membrane extension
  • Can be in globular or filament form
• Filament networks (bundles) for maintaining cell structure
• Myosin motors for contractility and vesicle transport
• Actin bundles and intermediate filaments for cell adhesion
• Lamin network (intermediate filament) for nuclear structure
  • Inside of nuclear envelope
• Microtubules work with other cytoskeleton components for proper function

Actin Filaments and Microfilaments

• Structure
  • Thinnest filament, ~6 nm
  • 15% of total protein of a cell
  • G-actin and F-actin types
  • Actin filament has a + and a – end and is dynamic – forming when needed and dissociating when not needed
    • Most material joins on the + end when forming and most material leaves on the – end when dissociating
    • Actin grows fast on + end and very slowly on - end
  • Three classes of actin – α in muscle cells and , β and γ in non-muscle cells
• Actin-Binding Proteins
  • Necessary for proper function – defects can lead to disease
  • Fimbrin
    • Microvilli, stereocilia, adhesion plaques, yeast actin cables
    • Help actin filaments bind side-by-side to form bundle
  • α- Actinin
    • Filopodia (extension of cell membrane), lamellipodia, stress fibers, adhesion plaques
    • Help actin filaments bind side-by-side to form bundle
  • Spectrin
    • Cortical networks
    • Allows cell to retain flexibility necessary for cells such as blood cells to move through the thin corridors of capillary systems
  • Dystrophin
    • Muscle cortical networks
    • Muscular dystrophy occurs when dystrophin is defective
  • Filamin
    • Filopodia, pseudopodia, stress fibers
  • Fascin
    • Filopodia, lamellipodia, stress fibers, microvilli, acrosomal process
  • Villin
    • Microvilli in intestinal and kidney brush border
  • Cofilin
    • Dissociation from – end
  • Severin and Gelsolin
    • Severing, capping + end
• Mechanism of Assembly and Disassembly
  • Globular actin molecules form a dimer, then a trimer, and continue to polymerize to form the helical actin filament through the nucleation process
  • Rapidly growing and slow growing ends
  • G-actin, Ca2+ and ATP needed
• Actin Bundles and Filaments – formed by cross-linking proteins
  • Contractile, i.e. cleavage furrow (myosin) – can be used to push things around
  • Gel-like network, i.e. cell cortex (filamin)
  • Parallel, i.e. core of microvilli (villin, fimbrin)
  • Focal contact for cell attachment, i.e. stress fiber (α-actin, talin, vinculin)
• Microvilli
  • Finger-like projections of surface of intestinal epithelium with variable length (0.5-10 μm)
  • Brush border of intestinal epithelium – increases surface area to transport nutrients by extending plasma membrane using actin filaments
  • Contains core actin filaments, membrane linkage, cross-liked by fimbrin and villin
• Action of Drugs
  • Cytochalasins – used to prevent further polymerization of actin by binding to the ends of actin filaments
    • From various fungi
  • Phalloidin – binds to actin filaments and stabilizes against depolymerization
    • From mushrooms, very potent poison

Intermediate Filaments

• Provide a strong 3-D structural support of cell
  • Kind of like bones of a cell
  • Anchor the nucleus in place
  • Connect the cell membrane and cytoskeleton
  • Form the framework for the maintenance of the nuclear envelope and nuclear structure
• Structure
  • Two of them join to form a dimer, then two dimmers form a protofilament, elongate, and wrap up into an intermediate filament
• Principle types of intermediate filament proteins
  • Cytoplasmic Intermediate Filament – can be used to identify the source of cancer primaries
    • Keratins – in epithelial, hair, nails
    • Desmin – in muscle cells
    • Vimentin – in fibroblast and leukocytes
    • Glial Fibrillary Acidic Protein – in astrocytes, Schwann cells
    • Neurofilaments – neurons
  • Nuclear Intermediate Filament
    • Nuclear Lamins – all nucleated cells
      • Control assembly of nuclear envelop by organizing perinuclear chromatin
      • Type B bines with inner nuclear member
      • Type A and C bind with B
• Functions
  • Very Stable, distribute tensile strength across cell and tissue
  • No polarity (+ end or – end) because they do not assemble or disassemble easily

Microtubules

• Structure
  • Has a + and – end polarity
• Types of Microtubules
  • General Cytoplasmic
  • Central pair of cilia and flagella
  • Fused
• Microtubule Associated Proteins – MAPs
  • Can organized microtubules and affect their stability
• Assembly and Disassembly of Microtubules
  • Hollow tube composed of 13 protofilaments made from tubulin molecules form to make a microtubule
  • Can be fused in doublets (cilia, flagella) or triplets (basal bodies, centrioles)
  • Have a rapid growing end and slow growing ends
  • Mg2+ and GTP are needed
  • Some MAPs are needed also
  • Formed by α and β tubulin molecules into protofilaments arranged in a cylinder
• Motor Proteins
  • Kinesin – Transport vesicles to + end
  • Dynein – Transport vesicles to – end
    • Present in cilia and flagella
• Action of Drugs
  • Colchicine, Colcemid – inhibit addition of tubulin molecules to microtubules, leading to microtubule depolymerization
  • Vinblastine, Cincristine – induce the formation of paracrystalline aggregates of tubulin
  • Paclitaxel – Stablizes tubulin

Nucleus

• Bound by nuclear envelope, contains nucleolus, chromatin, and nucleoplasm
  • Varying different shapes and sizes
• Nucleus is the information center of the cell
• The Interphase Nucleus
  • Distinct nuclear envelope, chromatin (light, filamentous material), nucleoplasm, and some centrosome and centriole (microtubules) structures outside of the nucleus
• Nuclear Envelope
  • Double lipid bilayer with pore-complexes in envelope
  • Continuous outer membrane with rough ER
  • Inner membrane has nuclear lamin (30-100 nm)
• Nuclear Pore Complex (NPC)
  • 80-100 nm, composed of proteins called nucleoporins
  • Necessary for precise transport of specific proteins in and out of nucleus
    • Larger molecules must interact with other proteins with nucleoporins for transport
  • Has 3 rings: cytoplasmic, middle, and nucleoplasmic
  • Cytoplasmic and middle rings are connected by 8 spoke-like structures
  • Nucleoplasmic ring has a protruding nuclear basket
  • Middle ring contains a central transporter
    • FG-nucleoporins line the central transporter channel and transport all macromolecules
  • Exportins – transport macromolecules to cytoplasm
  • Importins – transport proteins to nucleus
• Nucleolus -- ribosome production factory
  • Fibrillar center of inactive DNA
  • Pars fibrosa – RNA
  • Pars granulose – maturing ribosomal units
  • Nucleolar matrix – has some nucleolus organization proteins
• Chromatin – made up of DNA
  • Hematochromatin
    • Closer to nuclear envelope and is very dense
    • Not transcriptionally active
    • Low protein synthesis
  • Euchromatin
    • Much lighter than hematochromatin
    • Transcriptionally active
    • Meaning euchromatin cells are producing more protein than hematochromatin
• Centrosome and Centriole
  • Centrosome
    • Microtubular organizing center (MTOC)
    • Some cells may not have centriole but will have centrosome instead
    • Centrosome is composed of the centrioles and surrounding pericentriolar material consisting of γ-tubulin ring complex, pericentrin, and other microtubule-nucleating macromolecules
    • Centrosomes assist in the formation and organization of macrotubules and self-duplication before cell division
  • Centriole
    • Centrioles in the normal in interphase cell are found in pairs at right angles to each other
      • Centrioles are composed of nine triplets of microtubules arranged around a central axis.
    • Centrioles when replicating will separate and bind with tubulin and begin forming microtubules at a right angle
    • Centrioles function in the formation f the centrosome and guide the formation of the spindle apparatus, cilia, and flagella
  • Microtubules (spindle fibers)
    • Kinetochore microtubules – connecting from centrosome to chromosome (“captures” chromosomes)
      • The location where the kinetochore microtubule attaches to the chromosome is called the kinetochore
    • Polar microtubules – from centrosome pole to pole
    • Astral microtubules – from centrosome and extend outwards
    • Microtubules grow outward from the poles

Mitotic Chromosome Movement

• Kinetochore microtubules shorten from – end
• Motor proteins on polar microtubules push them apart as polar microtubules elongate from + end
• Astral microtubules shorten from – end while motor proteins also help pull
• All three actions help separate chromosomes in anaphase
• The assembly and disassembly of microtubules is what allows the movement of chromosomes (an any other organelles in the cell)
  • Control of assembly and disassembly controlled by availability of Mg2+ and GTP

Objectives

Objectives - Cell Structure & Function II.doc

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